The treatment of different solid, liquid and gas wastes and hazardous residues represents a very important environmental technology, particularly treatment of industrial flue gases and exhaust gases from engines, due to their pollution of the air by NO.sub.x, SO.sub.x, soot, hydrocarbons, volatile organic compounds, etc. The most common methods are based on chemical reactions, pyrolytic combustion and different catalyst filters. Chemical filters are often used for treatment of flue gas from industrial and energy production. They can treat large throughputs of gases, but their substantial disadvantage is formation of byproducts (e.g., NH.sub.3, CaO.sub.2, etc.). Catalysts are regularly used for instance in car exhaust after-treatment. Typical catalyst materials are based on Pt or different metal oxides e.g., V.sub.2 O.sub.5, WO.sub.3, TiO.sub.2, ZnO, etc. Despite their simplicity the catalyst filters often exhibit limited efficiency, particularly in Diesel engines, and strong temperature dependence.
Contrary to the conventional chemical and pyrolytical technologies, the gas discharge plasma treatment often leads to apparently more efficient transformation reactions with little or no undesirable byproducts. This is given by the possibilities either to achieve extremely high gas temperatures in the plasma, or to generate very reactive species through dissociation, activation or ionization of gaseous and volatile components in the plasma. Pyrolytic effect of plasma can be achieved for instance in atmospheric pressure arc torches at high powers generating large current densities in the ionized gas. These plasmas are called equilibrium or thermal plasmas and they are characterized by high collision frequencies, which equalize energy of all particles present.
The energies (temperature) of electrons and ions are practically the same and they approach the energy of neutral species in the partially ionized plasma. The gas temperature is high, up to several thousands degrees centigrades, hence the term "thermal" plasma. The thermal plasma is very suitable for irreversible thermal treatments e.g., for combustion of solid and liquid wastes, or in plasma metallurgy, etc. However, due to heating of all species evenly the energy consumption in thermal plasmas is typically high. At equilibrium conditions the gaseous products of plasma chemical reactions are often unstable due to almost equivalent probability of reverse chemical reactions. The efficiency of thermal plasmas for plasma chemical treatment of gases is therefore low in comparison with so called non-equilibrium plasmas. Non-equilibrium plasma can be simply generated at reduced gas pressure. Then the frequency is lower and due to different electron and ion masses (the electron mass is 9.11.times.10.sup.-31 kg, the proton mass is 1.67.times.10.sup.-27 kg) the electrons can acquire much higher kinetic energy than ions. When the plasma is generated by a very high frequency electromagnetic field, the power is acquired mainly by mobile electrons, while heavy ions are not able to even follow changes of the field and move only due to their thermal energies similar to the rest of the neutral gas. This leads to the plasma in which the chemical reactions are very effective while the bulk of gas remains relatively cold. The non-equilibrium plasma is therefore often noted as "cold" plasma. Interactions of high-energy electrons with gas can produce extremely reactive atoms and radicals, which are able to generate subsequent chemical reactions not available at normal conditions. A very high plasma chemical activity of such plasmas can be utilized in different applications (see for instance a pioneering work of F. K. McTaggart: "Plasma Chemistry in Electrical Discharges", Elsevier, Amsterdam, 1967). However, due to necessity of pumping systems, the reduced and low-pressure non-equilibrium plasmas are not utilized in industrial scale waste treatments.
An emerging technology in this field is non-equilibrium plasma of atmospheric pressure. The degree of energy non-equilibrium is somewhat lower than in low-pressure plasmas and strongly depends on the arrangement of individual reactors. However, the absence of pumps simplifies all systems substantially and in principle allows their immediate application for large gas throughputs. On the other hand the efficiency of known systems is still not high enough for their utilization in an industrial scale. The non-equilibrium conditions at atmospheric pressure can be achieved in several ways. The most direct way is an injection of an electron beam into the gas. The high power electron beam interacts with the gas and generates non-equilibrium plasma along its penetration depth. A serious disadvantage of the method is that it acts only in a limited space and that the penetration depth at atmospheric pressure is rather short. Moreover the electron gun system is quite complicated and expensive and it does not show satisfactory high energy efficiency. The more common way of generation of non-equilibrium plasma for gas treatment is a high voltage breakdown of the gas in form of many filamentary current paths plasma streamers. A typical representative is corona discharge between sharp edged or sharp tipped electrode (cathode or anode) and the grounded counter electrode. At a high frequency generation (orders of 1 kHz up to more than 1 MHz) and high voltage (10-30 kV), the counter-electrode may be covered by a dielectric wall (barrier) and then the system works with a barrier (also "silent") discharge.
Another very sophisticated reactor is composed from an axial system of at least one pair (typically three pairs) of knife sharp electrodes facing each other by sharp edges and connected to the high voltage generator, see for instance French patent No. 2639172 (1988) to H. Lesueur et al.. Arc streamers between related electrodes are gliding over the sharp edges between opposite electrodes and also around the axial system of electrodes following the phase movement in a 3-phase generator.
The generation of local current streamers in all systems mentioned above provides local non-equilibrium plasmas when the rest of the gas remains "cold". A great advantage found in coronas and barrier discharges is a pulsed generation, see U.S. Pat. No. 5,603,893 (1997) to M. Gunderson et al. The high power pulse allows quick pumping of the power into the streamers causing strong non-equilibrium plasma usually at the beginning of the pulse with the relaxation into the equilibrium conditions depending on both the pulse shape and the duty cycle. Although all these systems in both stationary and pulsed regimes are very advanced, the region where the gas interacts with streamers is not dense enough or of sufficient volume (bulk) for treatment of all the gas passing the reactor zone. Generation of bulk atmospheric pressure non-equilibrium plasma is possible by microwave power. This type of generation is based on very high frequency (typically 2.45 GHz and more) connected with the pumping of power directly into electrons. Although the advantage of microwave systems is a denser plasma volume than in the case of streamers, substantial disadvantages are low efficiency and short lifetime of microwave generators and limited plasma dimensions (related to the size of the wave-guide). Therefore, in spite of their discrete streamer character the most serious candidates considered for industrial atmospheric pressure plasma treatment of gas are still the pulsed corona, pulsed barrier discharge and the gliding arc.
Non-equilibrium plasma with very high degree of activation can be generated at reduced pressures by hollow cathodes. Since their discovery by F. Paschen in 1916, the hollow cathodes have been used for quite a long time as the sources of intense light for atomic spectroscopy. Experiments of Little and von Engel in 1954 revealed clearly the principle of an exceptionally high plasma density and activation in hollow cathodes through so called "hollow cathode effect". This effect is based on a special geometry in the cathode, where the opposite walls have the same electric potential with respect to a common anode. In a direct current (DC) arrangement of the diode gas discharge the cathode wall is covered by cathode dark space in which electrons emitted from the cathode surface are accelerated towards the anode. In the suitable "hollow geometry" when the cathode fall regions of opposite cathode surfaces are close to each other, the emitted electrons can meet the opposite region with the equal opposite electric field of the cathode fall region. Electrons are therefore repelled back and undergo oscillations called a "pendulum electron motion". This kind of motion leads to confinement of electrons and intensifies their interactions with the gas present in the hollow cathode, which promotes dramatically overall efficiency of the ionization and subsequent effects, terminating in a very high density active plasma. Hollow cathodes are capable of production of electron beams having energies comparable to the cathode fall potential and, moreover, their extraordinary abilities are already known also for generation of different plasma chemical reactions in gases for applications mainly in surface processing. Moreover, due to non-Maxwellian energy distributions and existence of high energy electron populations the hollow cathode discharge emits an intense radiation in UV (.ltoreq.300 nm) and VUV (.ltoreq.200 nm) regions capable to breakdown most chemical bonds and to induce different photochemical reactions.
Compared to the DC generation of the hollow cathode, an alternating current (AC) and particularly a radio frequency (RF) generation provides a number of advantages. In this case the most positive body in the system is the gas discharge plasma outside the cathode. This plasma can substitute a "virtual anode" and is naturally flexible with respect to any cathode geometry. As a consequence, the hollow cathode behaves as a unipolar discharge (see e.g. L. Bardos et al., Thin Solid Films 1987, or recent reviews by L. Bardos et al., Surf. Coat. Technol. 1996 and 1997). The AC generation has also an important thermal stabilizing effect.
To satisfy conditions for the hollow cathode effect the distance between the opposite walls in the cathode must be in suitable relation with the thickness of the cathode fall, or the space charge sheath in the RF case, to enable the electron exchange. One of the most important parameters is the gas pressure, which affects both the thickness of the cathode regions and the electron recombination by collisions. Therefore the hollow cathodes are operated typically at reduced pressures, below the order of 10 Torr. Very recently the cylindrical closed end DC molybdenum hollow cathodes with diameters below 0.1 mm were reported to work at air pressures of 350 Torr.apprxeq.50 kPa (K. H. Schoenbach et al., Appl. Phys. Lett. 1996). Similar DC cathodes with diameters of 0.2-0.4 mm and 0.5-5 mm in depth were reported to sustain the nitrogen discharge even beyond the atmospheric pressure (&gt;750 Torr.apprxeq.100 kPa), see J. W. Frame et al, Appl. Phys. Lett, 1997. Atmospheric pressure xenon discharges were generated in a 0. 1 mm diameter dc hollow cathode by A. Al-Habachi and K. H. Schoenbach (Appl. Phys. Lett. 1998). These DC hollow cathode discharges were able to produce intense UV and VUV excimer emissions. Arrays of the closed end micro-hollow cathodes have been used in UV lamp applications, see U.S. Pat. No. 5,686,789 (1995) to K. H. Schoenbach et al.
No works, arrangements or results have yet been found published regarding the utilization of hollow cathodes for treatment of a flowing gas or for passivation of polluting and toxic gas mixtures neither at atmospheric pressure nor at reduced pressures. Moreover there are no works known yet reporting on AC generated hollow cathodes or related reactors for this purpose at atmospheric pressure.